There exists a long felt need for a technique for performing Fourier transformation analysis of incoherent optical data in real time. In order to process the image of an object scene using conventional techniques, the image must first be received by a transducer and converted to an appropriate form. For electronic processing, the image data is converted into voltage variations and for coherent optical processing, it is converted into amplitude transmittance variations. This conversion step tends to limit the operating speed of the processing system. If a system is capable of processing incoherent optical data on the other hand, the conversion step is eliminated and the processing can be performed at the speed of light.
Fourier transformations can be performed electronically using a suitable computer and software. The processing by a computer is usually serial in nature and the processing speed is very limited. The use of an array processor increases the amount of parallelism and the processing speed. True real time (speed of light) processing, however, is still not possible with this approach. Coherent optical processors can perform the Fourier transformation in real time. To process the image of an incoherently illuminated object scene, however, requires an incoherent to coherent optical conversion using a spatial light modulator which limits the system throughput, reduces the dynamic range and introduces nonlinearity.
One dimensional processing using bulk acoustical modulators or SAW devices on the other hand are limited in the space-bandwidth product by physical constraints such as transducer frequency response, acoustical velocity and acoustical attenuation. Moreover, processing with acoustical modulators is limited to serial inputs. To be able to process parallel spatial inputs such as images, a spatial-to-temporal conversion is needed.
Incoherent optical processing techniques such as OTF synthesis permit the use of incoherent optical inputs but their applications are limited to a rather restrictive class of operations. Complex Fourier transformations, for example, has not been demonstrated. The optical vector-matrix multiplier operates on discrete sampled signals. The difficulties in constructing and aligning a large two-dimensional mask and the limits on the densities of these masks restrict the space-bandwidth product that can be practically achieved. The relative complexity of the system also diminishes its attractiveness.
This invention is based on the physical principle described by the well known Van-Cittert Zernike theorem. It states that a Fourier transformation relationship exists between the intensity distribution of a quasimonochromatic, spatially incoherent source and the mutual coherence function at far field. This relationship is the basis for the Michelson stellar interferometers. Thus, by measuring the mutual coherence function at far field (the far field condition can be satisfied with the use of a collimating lens), the complex Fourier transform of an incoherent optical input can be obtained. The problem is devising a practical means to measure and display the complex mutual coherence function in real time.
With the Michelson stellar interferometer and other similar interferometric imaging systems, the spatial frequencies are sampled by physically varying the separation of the detecting apertures. Such a sequential measurement scheme is too slow for use in a real time processing system. The requirements that the input be quasimonochromatic also makes the system very inefficient for inputs that are polychromatic.
The present invention is directed to solving one or more of these problems encountered with the systems utilizing prior art techniques.